1139
The Characterization of Dimeric Dithiolene Complexes' A. L. Balch,2I. G . Dance, and R. H. Holm3 Contribution from the Departments of Chemistry, University of California, Los Angeles, California, and University of Wisconsin, Madison, Wisconsin. Received August 16, 1967 Abstract: Several series of bis(dithio1ene)cobalt and -iron complexes have been prepared. Their spectral, mag-
netic, and electrochemical properties are best interpreted and systematized when the complexes are considered as members of the general series [M-S4I20,[M-S412-, [M-Sr12-2, [M-S4]+. By utilizing the ligands toluene-3,4-dithiolate, maleonitriledithiolate, and bisperfluoromethyl-1,2-dithiene,partial or complete electron-transfer series of this type have been established by preparation and/or polarographic detection of some or all of the complexes in each. The complete series of dithietene complexes, [MS4C4(CF3)4]20, [MS4C4(CF3)4]2-,[MS4C,(CFa)4]2-2, [MS4C4(CF,),]-2 (M = Fe, Co) has been demonstrated, and all four members of the cobalt series have now been isolated in this and previous work. The paramagnetic complexes usually do not obey the Curie law, and the temperature dependence of their magnetic moments is indicative of antiferromagnetic spin exchange. The results of this work, together with recent X-ray structural determinations, reveal that the bis-chelate dimer structure represents a third clear-cut stereochemical pattern in the chemistry of dithiolene complexes, and that, in particular, it resembles the other two (planar, trigonal prismatic-octahedral) by supporting well-defined electron-transfer series.
T
he chemistry of bis- and tris-chelate complexes formally derived from cis-1,Zdisubstituted ethylene1,2-dithiolates (1) and 1,2-dimercaptobenzene (2) and its ring-substituted variants, hereafter referred to as dithiolene c o m p l e x e ~ , has * ~ ~progressed to a point such that two basic stereochemical patterns in particular are known to be of general significance. X-Ray structural studies have established the square-planar stereochemistry of [NiS4C4Ph4],6[Ni(mnt)&,' [Ni(mnt)2]-z,8 [Co-
1
2
(mnt)z]-2,9 [C~(tdt)~]-,~O and [Cu(mnt>,]-. l 1 Isomorphism relationships indicate planar structures for [M(mnt)z]-2 (M = Rh, Pt, Cu) as we11.12 On the basis of (!) Publication No. 2092 from the Department of Chemistry, University of California, Los Angeles, Calif. 90024. (2) National Science Foundation Predoctoral Fellow, 1963-1966; University of California. (3) Alfred P. Sloan Foundation Fellow; Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Mass. 021 39. (4) We subscribe to the term "dithiolene," coined by others,' as a generic name for ligands of the types RZCZSZ and 1,2-CsH& in complexes 1 and 2 of differing total charge z . By so doing formal oxidation states of the ligands and metal are not implied by the nomenclature. Classical oxidation states are for most purposes misleading when defined for [M-S#,-l and related species; cf. F. Lalor, et al., J . A m . Chem. Soc., 89, 2278 (1967), and R. H. Holm, et al., ibid., 89, 2866 (1967). The standard abbreviations mnt for (NC)GSZand tdt for 4-CHaCsHaSn are used throughout the text. (5) J. Locke, J. A. McCleverty, E. J. Wharton, and C. J. Winscom,
Chem. Commun., 677 (1966). (6) D. Sartain and M. R. Truter, J . Chem. Soc., Sect. A , 1264 (1967).
For preliminary X-ray data on [N~SIC~HI], cf. E. Hohne, P. Prokop, and E. Hoyer, Z.Chem., 6,71 (1966). (7) C. J. Fritchie, Jr., Acta Cryst., 20, 107 (1966). (8) R. Eisenberg and J. A. Ibers, Inorg. Chem., 4,605 (1965). (9) J. D. Forrester, A. Zalkin, and D. H. Templeton, ibid., 3, 1500 (1964). (10) R. Eisenberg, 2. Dori, H. B. Gray, and J. A. Ibers, submitted for publication. (1 1) J. D. Forrester, A. Zalkin, and D. H. Templeton, Inorg. Chem.,
3, 1507 (1964). (12) (a) E. Billig, S. I. Shupack, J. H. Waters, R. Williams, and H. B. Gray, J . A m . Chem. Soc., 86, 926 (1964); (b) H. B. Gray, R. Williams, I. Bernal, and E. Billig, ibid., 84, 3596 (1962); (c) E. Billig, R. Williams, I. Bernal, J. H. Waters, and H. B. Gray, Inorg. Chem., 3, 663 (1964).
the current body of chemical and physical evidence, it appears quite likely that all [CO-S~]-~,[Ni-S4]-2*0,[Pd[Pt-S4]-2, [Au-S4]-, and [CU--S~]-~~complexes of types 1 and 2 exist as planar monomeric species in crystalline and solution phases. The second stereochemical pattern of considerable importance is the trigonal prism which is found in the neutral tris complexes [MS6C6Ra]and [M(tdt),]. 13,l 4 Thus far this structure has been definitely established for [ R e s ~ C g P h a ] ,[MOSaCaHe],14b ~~~ and [vs6c6Ph8],14c and may also extend to the general group of neutral complexes [MS6C6(CF3)6] l6 and to certain anionic comThe occurrence of the more usual tetrahedral and octahedral coordination polyhedra and, more significantly, the generality of these structures as a function of metal and total charge z have not yet been established in detail. It is likely that [Zn-S4]-2 species, such as [Zn(tdt)2Ia2 l6 and [Zn(mnt)2]-z,12c are tetrahedral. The only proven structures of these types are those of [Fe(mnt)3]-2, in which a nearly octahedral coordination of sulfurs around the iron was found, l7 and [V(mnt)3]-2, which has only Cz symmetry and whose coordination polyhedron may be considered a very distorted octahedron. l8 In this paper we present evidence bearing on the occurrence and generality of a third important stereochemical pattern in dithiolene chemistry, the bis-chelate dimers [M-S4]zE. The first indication of interionic or (13) E. I. Stiefel and H. B. Gray, J . Am. Chem. Soc., 87, 4012 (1965); E. I. Stiefel, R. Eisenberg, R. C. Rosenberg, and H. B. Gray, ibid., 88, 2956 (1966); G. N. Schrauzer and V. P. Mayweg, ibid., 88, 3235 (1966). (14) (a) R. Eisenberg and J. A. Ibers, Inorg. Chem., 5, 411 (1966);
(b) A. E. Smith, G. N. Schrauzer, V. P. Mayweg, and W. Heinrich, J. A m . Chem. Soc., 87, 5798 (1965); (c) R. Eisenberg, E. I. Stiefel. R. C. Rosenberg, and H. B. Gray, ibid., 88, 2874 (1966); R. Eisenberg and H. B. Gray, Inorg. Chem., 6, 1844 (1967). (15) A. Davison, N. Edelstein, R. H. Holm, and A. H. Maki, ibid., 86, 2799 (1964). (16) W. H. Mills and R. E. D. Clark, J . Chem. Soc., 175 (1936). (17) I. Bernal and A. Sequeira, Abstracts, American Crystallographers, Washington, D. C., Aug 1967; I. Bernal, private communica-
tion. (18) E. I. Stiefel, Z. Dori, and H. B. Gray, J . A m . Chem. Soc., 88, 3353 (1967).
Balch, Dance, Holm / Dimeric Dithiolene Complexes
1140
dimers. The apical M-S distance in [Ni(mnt)$ is indicative of a nearly limiting case of weak yet discernible dimerization. Further supporting evidence for the generality of the dimer structure is found in the isomorphism of [FeS4C4(CF&I2 and its cobalt anai 0 g ~ 5 ~and 2 ~ the possible isostructural relationships of [MS4C4(CFJ4]and [MS4C4Ph4](M = Fe, Co). 26 In this work we show that the basic dimeric structure of Figure 1 resembles the planar and trigonal prismatic (or the latter in combination with octahedral) stereochemistries in that it supports electron-transfer reactions in which the essential stereochemistry is unchanged as Figure 1. Idealized representation of the structures of dimeric dithiolene complexes, based on the structures of [ C O S ~ C ~ ( C F ~ ) ~ ] ~the , ~ Ototal charge on the complex is varied in the series (n-Bu4N)~[Fe(rnnt)~]~, and (n-Bu4N)z[Co(o-S2CaClr)z1~. 24 [MS4C4R4lZz (M = Fe, Co) and [Fe(tdt)2]2z. We have previously communicated sufficient information to demonstrate the existence of electron-transfer series of dimeric dithiolenes, 25 and more recently conductivity intermolecular interaction of dithiolene complexes was measurements have been published which support the found in the reduced magnetic moments of some crystalA presence in solution of certain [ C O - S ~ ] ~species.27 -~ line [M-SJ salts (M = Ni, Pd, Pt), which in solution gave more complete body of results is presented here which moments expected for a doublet ground state.lg [Coserves to define the electron-transfer series of dithiolene S4C4(CFJ4]was found to be a diamagnetic solid and dimers and to characterize individual members of each dimeric in carbon tetrachloride solution. 2O Shortly series. More detailed interpretations of the magnetic thereafter, it was demonstrated that the magnetic susproperties of individual dimers and their solution ceptibilities of crystalline salts of [M(mnt)a]- (M *= equilibria will be presented subsequently. 28 Ni, Pd, Pt) could be interpreted in terms of pairwise interaction of doublet-state anions.2 The [Ni(mnt)& anions have been shown to be weakly dimerized in the Experimental Section triphenylmethylphosphonium salt. Preparation of Compounds. [FeSrC4(CF3)4 12. This preparaFinal proof of the existence of discrete dimers in the tion follows the previously reported procedure for “[FeSaCs( c F 3 ) ~ ] ” . ~Iron pentacarbonyl (5.8 g, 30 mmoles) and 14 g (62 crystalline phase has been provided by the X-ray studies mmoles) of bisperfluoromethyl-1,2-dithieteneZQ(freshly distilled, of [ C O S ~ C ~ ( C F ~( n) ~- B] ~ ~, ~N~) d F e ( m n t ) ~and ] ~ , ~(n~ bp 96-98”) were refluxed in 600 ml of dry n-pentane under a nitroB U ~ N ) ~ [ C O ( C ~ C I ,2S4 ~ )The ~ ] ~ .dimer of each of these gen atmosphere. The reaction mixture became very dark rea within species has the same essential structure, which is shown several hours. Refluxing was continued until the solution color was blue-black (15-20 hr) The warm solution was filtered to rein Figure 1. Each dimer is formed by the lateral intermove a small quantity of insoluble material. The filtrate, protected action of two bisdithiolene units such that the metal from moist air, was cooled to -10”. The crystals which slowly assumes square-pyramidal coordination by forming, in deposited were collected and washed with pentane. A yield of addition to the four basal M-S bonds in the monomeric about 2 5 z results. The product was recrystallized several times unit, a somewhat longer apical M-S bond. The from carbon tetrachloride to yield fine black crystals: mp 190-191 (sealed tube). presently available detailed structural data, sumAnal. Calcd for C12Fl8SaFe: C, 19.65; H, 0.00; S, 26.20. marized in Table I, reveal the close similarity of the Calcd for C8FlzS4Fe: C, 18.90; H, 0.00; S, 25.24. Found: C, 18.78; H,0.03;S,25.70. Table I. Brief Structural Comparison of Dimeric Dithiolenes Distance,
A
Basal M-S (av) Apical M-S M-S4 planee M*..M
[CoS,C4[Fe[Co(o[Ni* SzCBC14hl~-2 e (mnthl(CF3)4]za( m n t ) ~ ] ~ -~
2.16 2.38 0.37 2.78
2.23 2.46 0.36
. . */
2.18 2.40 0.26 3.10
2.15 3.59
...
4.40
4 Data from ref 22. * Bu4N+ salt; data from ref 23. e Bu,N+ salt; data from ref 24. d(Ph3PMe)+salt; data from ref 7; the shortest set of interionic distances given. Evertical distance of metal from S4 plane. f Not reported.
coordination polyhedra, supporting the assumption made in this work that the structure shown in Figure 1 may be taken as the prototype for all bisdithiolene (19) A. Davison, N. Edelstein, R . H. Holm, and A. H. Maki, Inorg. Chem., 2, 1227 (1963). (20) A. Davison, N. Edelstein, R. H. Holm, and A. H. Maki, ibid., 3, 814 (1964). (21) J. F. Weiher, L. R . Melby, and R. E. Benson, J . Am. Chem. Soc., 86, 4339 (1964). (22) J. H. Enemark and W. N. Liuscomb. Inorg. Chem.. 4. 1729 (1965). (23) W. C. Hamilton and I. Bernal, ibid., 6, 2003 (1967). (24) M. J. Baker-Hawkes, Z. Dori, R. Eisenberg, and H. B. Gray,
submitted for publication.
In 1 hr a solution of 1.597 g (3.125 [(CzH5)4N][FeS4Cr(CF3)4]z. mmoles) of [NiS4C4(CF3)4]1Q in 150 ml of dry dichloromethane was added to a slurry of 3.986 g (6.245 mmoles) of [(CZHS)4N]~[FeS4Cr(CF3)4]Pin 200 ml of dry dichloromethane. After this mixture was refluxed for 6 hr, it was filtered while still hot. The black crystalline material collected was washed with dichloromethane and vacuum dried. The yield of crude product was 3.32 g (9373. Purification was accomplished by Soxhlet extraction with dry dichloromethane, yielding black crystals, mp 259”dec. Anal. Calcd for Cz4HzaF~4NS8Fe2:C, 25.14; H, 1.76; F, 39.77; N, 1.22; S, 22.37. Found: C, 25.28; H,1.52; F, 40.72;
N,0.99;S, 22.18. This compound has also been prepared in an entirely analogous procedure but by use of [FeS4C4(CF3)4]z and [(CzHa)rNl~[FeS4C4(CFS)& in an exact 1 :1 mole ratio. Black crystals were isolated, mp 261-262”dec. [(n-C4H~)4N][Fe(SzCd3aCH&]~.A solution of 0.1511 g (1.190 mg-atoms) of iodine in 70 ml of dichloromethane was added slowly with stirring to a solution of 1.356 g (2.234mmoles) of [(C4H&N][Fe(SzCeH3CH3)z]ao in 30 ml of dichloromethane. A steel blue color (25) A . L. Balch and R. H. Holm, Chem. Commun., 552 (1966). (26) G. N. Schrauzer, V. P. Mayweg, H. W. Finck, and W. Heinrich, J . A m . Chem. Soc., 88, 4604 (1966). (27) A . Davison, D. V. Howe, and E. T. Shawl, Inorg. Chem., 6,458 (1967). (28) I. G. Dance and R. H. Holm, unpublished work. (29) C. G. Krespan, J . A m . Chem. Soc., 83. 3434 (1961). (30) R. Williams, E. Billig, J. H . Waters, and H. B. Gray, ibid., 88, 43 (1966).
Journal of the American Chemical Society / 90:5 / February 28, 1968
1141 developed as the iodine was added. On completion of the addition the solution was rapidly filtered into 250 ml of 2-propanol. The product precipitated as a black microcrystalline powder which was collected, washed with 2-propanol and pentane, and vacuum dried. A yield of 0.83 g or 95% was obtained. In solution the complex decomposes to yield an oily yellow precipitate and a wine-colored solution. Anal. Zalcd for C44H60NS8Fe~:C, 54.41; H, 6.23; N, 1.44; S,26.41. Found: C, 54.71; H, 6.26; N,0.83; S,26.79. [(n-C&)4N]~[FeS4C4(CF3)4]z.This compound was prepared in a manner similar to that for the tetraethylammonium salt.% The crude material separated as an oil. The aqueous reaction solution was decanted and the oil treated with a small volume of 2-propanol, which caused it to solidify. This solid was recrystallized from acetone-2-butanol. Black crystals were obtained, mp 182-183' (sealed tube). Anal. Calcd for C Z ~ H ~ ~ F ~ Z C, N S 38.40; ~ F ~ : H, 4.83; F, 30.37; S, 17.09. Found: C, 37.96; H, 4 53; F, 30.39; S, 17.24. [(n-C4HQ)4N]2[FeS4C4(CN)1]2. Preparation of this compound was effected by the previously published procedure for the tetraethylammonium sa1t.l' It was purified by careful recrystallization from acetone-n-heptane. Black crystals were obtained, mp 197-198" (sealed tube). Anal. Calcd for Cz4HseN5S4Fe: C, 49.81; H, 6.27; N, 12.10; S, 22.16 Found: C, 50.14; H, 6.46; N, 12.01; S, 22.09. [(CZHE.)~N][COS~C~(CF~)~]~. A filtered solution of 1.25 g (1.22 mmoles) of [ C O S K ~ ( C F ~in) ~400 ] ~ ml of dry dichloromethane was added slowly with stirring to a slurry of 1.49 g (1.17 mmoles) of [(CZH~)~N]Z[COS~C in~100 ( C ml F ~of ) ~dry ~ dichloromethane. The mxture was refluxed for 6 hr, and then the volume was reduced to 250 ml. After cooling, the black crystalline complex was collected by filtration and washed with dichloromethane to yield 2.35 g (88 %). Purification was achieved by Soxhlet extraction using dry dichloromethane. Anal. Calcd for C Z ~ H ~ ~ F ~ ~ NC,S ~25.00; C O ~ H, : 1.75; F, 39.55; N, 1.22; S, 22.25. Found: C, 24.96; H, 1.74; F, 42.09; N, 1.14; S, 22.25. [(n-C4H&N]z[CoS4C4(CF3)4]~.The preparation follows that of the tetraethylammonium salt.20 The crude product separated as an oil. Dichloromethane and water were added to the reaction mixture to produce two separate phases. The complex was extracted into the dichloromethane layer (green solution), which was separated, dried over sodium sulfate, and reduced in volume at room temperature until the product crystallized. This material was once recrystallized from acetone-2-propanol; black needles were obtained, mp 169-169.5'. Anal. Calcd for C Z ~ H , ~ F ~ ~ N S , C C ,O 38.24; : H, 4.81; S, 17.02. Found: C, 38.14; H,4.78; S, 16.82. Other complexes used in this study were available from earlier investigations or were prepared according to published procedue.lZo, 19-21,26,27,30 Physical Measurements. Polarographic measurements were obtained by use of the techniques and instrumentation previously described3 except that for measurements in dichloromethane solution a modified reference electrode was employed. The modifica(0.50 tion involves replacing ( ~ Z - C ~ H ~ ) ~with N P(n-C4H9),NC1O4 F~ M ) as supporting electrolyte in the reference electrode compartment and in the salt bridge connecting the reference electrode with the junction compartment (see Figure 1 of ref 31). This modification was made because it was observed that dichloromethane solutions of (n-C4H9)4NPF6decompose slowly with production of acid. This decomposition, presumably abetted by traces of moisture, was not sufficient to affect any of our previous polarographic measurements. Conductivities were measured using Spectrograde acetonitrile as solvent and a Serfass conductivity bridge. Mass spectra were recorded on a Picker MS-9 spectrometer by employing 70-ev ionizing electrons and direct insertion. Electronic spectra were obtained using a Cary Model 14 spectrophotometer. Magnetic measurements of solids and solutions were made by the Gouy method using HgCo(NCS), and freshly boiled, distilled water as the calibrants, respectively. For measurements of solids below room temperature, a cryostat capable of continuous temperature regulation from 77 to 300°K was employed.
(31) F. Rohrscheid, A. L. Balch, and R. H. Holm, Inorg. Chem., 5, 1542 (1966).
Results and Discussion Mass Spectra. In view of the tight dimeric structures established in the crystalline solids, we sought to ascertain if dimers remained intact upon vaporization. Data from the mass spectra of the only two complexes suitable for such a study, [ C O S ~ C ~ ( C Fand ~ ) ~[FeS4C4]~ (CF3)4]2, are given in Table 11. The most striking Table 11. Principle Metal-Containing Ions with m/e > 450 in Mass Spectra of [CoS4C4(CF3)4]~ and [FeS4C,(CF3)4]2 ~~
Ion
m/e for '6Fe
~
mje for W o
796 777 680 661
. ..
[MSsCizFi51f [MSeCizFial+ [MzSrCaFizl+ [MS4CsFizl+ [MSaCaFiiI' [MSGFio]+ [MS4CPgl+
677 658 564 508a 489 470 451
*..
511° 492 473 454
a Careful observation reveals no ions of half-integral mass in this region; thus the ions observed are not the doubly charged parent ions.
feature of these spectra is the observation of ions which indicate that the dimeric structure of these complexes persists in the gas phase. Both complexes exhibit a parent i ~ n , ~and z both appear to fragment similarly. Fragmentation of the parent ion via the process [MSaC4(CF3)alzC+[MzSsCs(CF3)61+ f SzG(CF3)z
is supported by observation of metastable peaks [for M = Co: calcd 620, found 620; for M = Fe: calcd 614, found 6141. Presumably this process results in the loss of one of the nonbridging ligands to produce a daughter ion with structure 3. This is further evidence of the strength of the bridging metal-sulfur bonds but is l&
3
a different decomposition pattern from that of [FeS4C4Ph4],, which is thermally induced.26 These results also confirm the stoichiometry of the iron complex, which was assigned a tris formulation in earlier work20 but has since been corrected.z5,26 The ions [MS6Cb(CF3)5C]+and [MS6C6(CF3)4CF2C]+ could result from the presence of some [MS6C6(CF&] as an impurity. However, we consider this to be unlikely because the mass spectra of the tris complexes of molybdenum and tungsten do not exhibit ions of this type but do show well-defined parent ions.32 N o [MS6C6(CF3)6]+ions are observed in the mass spectra (32) Mass spectra of the monomeric complexes [NiS4C4(CF&] and [PtSrC4(CFa)a] do not exhibit any ions with m/e greater than that expected for the (observed) parent ions: A. L. Balch, unpublished observations.
Balch, Dance, Holm
Dimeric Dithiolene Complexes
1142
of the cobalt and iron compounds. We suggest that [MS6C6(CF3)5C]+might result from either of the processes [Mz~6cdCF3)6]++[MS6C6(CF3)sCIf -I- MF3 [MzSaCdCF&.l++[MS&dCFa)8?
+ F3MSzCdCF3)z
but metastable peaks which could substantiate these ideas have not been observed, The first proposed fragmentation is similar to that observed for [Co(CO)S Z C Z ( C F ~ ) ZViz., ] ~ , ~ [Co&C6(CF3)6]+ ~ + [cO&cs(CF3)5CF]+ COFZ. Ions related by successive loss of fluorine are a prominent feature of these spectra. In this regard the observed fragmentation patterns are similar to those observed for a series of r-cyclopentadienyl complexes de34 rived from bisperfluoromethyl-1,2-dithietene. Spin-Spin Interactions in Crystalline Solids. Association of monomers to form dimers or higher aggregates can seriously perturb the magnetic properties of the monomers if exchange coupling is sufficiently strong. This is the situation found in salts of [M(mnt)& whose temperature-dependent magnetic susceptibilities have been successfully interpreted in terms of a thermal population of singlet and triplet spin states arising from antiferromagnetic coupling of two spindoublet metal atoms.21 From their magnetic properties we have obtained evidence that a number of iron and cobalt dithiolenes are not monomeric in the crystalline state. On the basis of X-ray evidence already cited, it is assumed that the most likely mode of association in the solid phases is dimeric, in which case the possible spin multiplicities of the various species are as follows,
+
s
[M-S~IZO
= 0, 1 , 2 ,
,..
S
[M-Salz=
3/z,
6Iz,.. .
[M-SaIz-' S = 0, 1, 2, . . .
The magnetic susceptibilities of the paramagnetic, potentially dimeric or polymeric, dithiolene complexes have been measured over the temperature interval 80-30OoK, and the full data and interpretations will be presented in a forthcoming publication. z8 The data given in Table I11 are adequate for the present purpose of detecting spin exchange, manifested by reduction of magnetic moments with decreasing temperature, and the attendant dimeric (or polymeric) structures of the complexes. Because of small spin-orbit coupling in the first transition series and the considerable covalency in the metal-ligand bonding, magnetic moments are not expected to decrease significantly below the spin-only values for the various spin states. Spin-doublet, -triplet, and -quartet dimers are then expected to have moments equal to or in excess of 1.73, 2.83, and 3.88 BM, or in terms of the monomeric units, 1.23, 2.00, and 2.74 BM, respectively, We find that the monomeric complex ( B ~ ~ N ) [ C o ( t d tobeys ) ~ ] the Curie-Weiss relation xMcor(T 14) = constant, and thus confirms the spintriplet ground state30 in the solid phase. In contrast, all complexes listed in Table I11 except (Et4N)[CoS4Cd(CF3),]2 do not folIow the Curie-Weiss law. Their moments decrease with decreasing temperature, reflecting antiferromagnetic coupling of spins between adjacent [M-S4] coordination units. The cobalt and iron dimers fall into five isoelectronic groups: [Fe-S4]z,0 [Fe-S4I2-, [Fe-S4]2-2 and [CO-S~]~, [CO-S~]~-,and [CO-S~]~-~. The detailed susceptibility
+
(33) R. B. King and T. F. Korenowski, Chem. Commun., 771 (1966). (34) R. B. King and M. B. Bisnette, Znorg. Chem., 6,469 (1967).
Journal of the American Chemical Society / 90:5
Table 111. Magnetic Moments o f Crystalline Solids
[CoS4C4Phdn 0 . 7 3 (293) (EtrN)[CoS4Ca(CFa)rlzb 1.91 (300), 1 . 8 3 (91) [F&GPh4ln 1 .63 (292), 0 . 7 7 (82) [FeS4C4(CF&1~ 1 . 5 9 (301), 1 , 1 3 (87) (Et4N)[FeS4C4(CF3)4Izb 2.08 (300), 1.74 (80) (Et4N)z[FeS4C4(CF3)41p 1.39 (299), 0.31 (87) ( B ~ ~ N ) z [ F ~ S ~ C ~ ( C F ~ )1~. 4I 2~ (301), 0 . 3 7 (90) ( E ~ ~ N ) z [ F ~ S ~ C ~ ( C N ) ~ I Z 1.67c(301)0.48(84) ( B u ~ N ) z [ F ~ S ~ C ~ ( C N ) ~ I Z 1 . 5 5 (299), 0 . 4 3 (81) (BurN)[Fe(&CsHsCH&] 1 .99d (299), 0 . 9 6 (80) (BurN)[Fe(o-SzCsH3CHs)z]zb 3 . 6 (300), 2.8 (80)
0 '11
0 0 '/z
0 0 0 0 0 %e
a Curie magnetic moment per metal atom, unless otherwise indicated, without inclusion of temperature independent pararnagnetism and corrected for diamagnetism. b Moment per dimer unit. 1.62 BM reported in ref 21. 2.07 BM reported in ref 30. e Moments indicate high population of excited spin state(s).
data for the iron complexes of the first three groups reveal that excited electronic states of spin degeneracy greater than that of the ground state are thermally populated in the 80-300°K range. Except possibly for [CoS4C4Ph4],, where the effect is very small, we have found no examples of cobalt dimers which exhibit populations of excited spin states. Both [ C ~ ( m n t ) ~ ]and ~ - ~ [CoS4C4(CF,)4]2-2 are diamagnetic in several different salts; this behavior does not, of course, prove their dimeric formulation for diamagnetic monomers [Co-S4]- are in principle possible. The diamagnetism35 of crystalline ( B U ~ N ) ~ [ C O ( ~ - S ~ Cknown ~ C I ~ ) to ~ ] ~contain , a dimeric dianion, z 4 together with the frequent insensitivity of magnetic properties (spin-Hamiltonian parameters and magnetic moments) of dithiolene complexes to ligand structure suggests that [Co-S4]- monomers would possess a spin-triplet ground state as does [C~(tdt)~]-. An X-ray study of the cobalt-mnt complex reported in progressz7should further clarify the structure-magnetism relationship of these diamagnetic cobalt species. Polarography. Establishment of Electron-Transfer Series. Polarographic measurements supplemented by synthesis and isolation of many of the complexes involved have clearly demonstrated that monomeric bisdithiolene complexes may exhibit electron-transfer series of two or three mernbersl6*l 7 , * 1,z4,27,36,37 represented generally by [M-S4]-'
e [M-Sa]- e [M-Sd]'
For [MS4C4R4I2complexes the ease of oxidation is strongly dependent on the R group, the order of oxidative stability for a given metal being CN > CF3 > Ph > H > alk~1.~7[M(tdt)z]zand [MS4C,Ph4]2species appear to have comparable oxidative ~ t a b i l i t i e s . ~All ~ three members have frequently been isolated or detected polarographically in Ni, Pd, and Pt series, whereas only the -2 -1 step has been reported in published investigations of the Cu and Au series. The polarographic data set out in Table IV present convincing evidence that dimeric complexes participate in electron-transfer reactions. Data are reported on all iron and cobalt complexes that have been isolated t o (35) M. J. Baker-Hawkes, E. Billig, and H . B. Gray, J . Am. Chem. Sac., 88, 4870 (1966).
(36) G. N. Schrauzer and V. P. Mayweg, ibid., 87, 1483 (1965). G. N. Schrauzer, ibid., 88,
(37) D. C. Olson, V. P. Mayweg, and 4876 (1966).
/ February 28, 1968
1143
Table IV. Polarographic Data for Cobalt and Iron Dithiolene Complexes in Dichloromethane"
-~
Complex
[COS~C~(CF~)~IZ (E~~N)[COS~C~(CF~)~IZ (BuaN)z[CoSaCa(CF3)4Iz (E~~N)z[COS~C~(CF~)~I (Bu~N)z[COS~C,(CN)~~Z (Bu~N)z[COS~C~(CN)~I (Bu~N)[CO(C+SZC~H~CH~)Z] [FeS4C4(CFd4I2 (Et4N)[FeS4C4(CF3)412 (BU~N)~F~S~C~(CF~)~IZ (BU~N)~[F~S~C~(CN)~I~
(Bu4N)[Fe(~-SzCBH3CHa)~12 (Bu4N)[Fe(o-S2CsH3CH3)21
2[MS4]-' Ei/z, v -0.14 -0.11 -0.09 -0.05 +O. 17 +0.24 -0.71
eid/Cp [MS~IZ-' pa/mM +34 1-41 40
+
(-)b
+35 - 17 f17
. . .d
. . .d
-0.60'
. . .d , J . . .d
-0.95'
$9.54 +15e
[MSdz-' E1jXy
v
eid/C,g [MS~IZpa/mM
+O. 52 +o. 54 $0.56 +O. 60 +1.03 +1.06
+I8 +22 - 17 (-)b
- 18 -9
c
- 19 +I9 - 16 - 17 +I7 -7
+0,'67 +O, 69 $0.71 +1.06 +O. 16 $0.18
[MSdzEli2,v +1.19 +1.23 +1.24 +1.29
e [MS& id/C,p pa/mM +20 -21 - 20
(-15
. . .c ...
C
. . .c
+1.24 $1.27 +1.27
- 19
+0.48 +O. 48
- 17
- 18 - 17
. . .c
-7
Using (n-Bu4N)C104as supporting electrolyte, a rotating platinum working electrode unless otherwise specified, and the reference electrode described in the Experimental Section. b The oxidative instability of this compound precluded determination of the diffusion current. c No wave corresponding to this process has been observed. d Very drawn out waves which may correspond to this reduction are observed on a platinum electrode in both dichloromethane and acetonitrile. E Obtained in acetonitrile solution with a dropping Hg electrode, saturated calomel electrode, and 0.1 M Pr4NC104as supporting electrolyte. Under these conditions the reduction of ( B U ~ N ) Z [ C O S ~ C ~ ( C F ~ ) ~ ] ~ (to [COS~C,(CF~),]-~) yields an id/C of 9.3 pa/mM. J Apparent reaction with Hg precludes measurement. Concentrations calculated on the basis of the formulas given in column 1. a
compare favorably with diffusion currents for one-elecdate with the ligand systems selected for study. In all tron-transfer reactions of monomeric species in dicases where two or more complexes appear to be related chloromethane solution which are listed in Table V. by simple electron-transfer reactions, this relationship is confirmed by the observation of the appropriate polarographic waves which occur at quite similar potentials regardless of which member is investigated. 38 This behavior is illustrated in Figure 2, in which are shown the actual polarographic traces obtained from dichloromethane solutions of ( B u ~ N ) ~ [ C O S ~ C ~ ( C F ~ ) ~ ] ~ and ( E ~ ~ N ) [ C O S ~ C ~ ( C F ~ ) ~ ] Z . '20 It is convenient to consider the polarographic behavior in terms of complexes having a single negative charge per metal because these species are common to every series of dimers studied. The general features of the polarography of this group of complexes are exhibited by ( B U ~ N ) ~ [ C O S ~ C ~ ( CThe F~)~ most ] ~ . striking feature is the appearance of a reduction wave with a diffusion current twice that of each of the two oxidative waves. This behavior is consistent with the occurrence of two oxidative processes which leave the dimer intact to produce, successively, [COS,C~(CF,),]~-and [CoS4C4(CF3)4]2. The reduction wave involves a two-electron transfer, yielding two [COS~C~(CF&]-~ ions. In the Comnt series the only isolable dimeric species, [ C ~ ( m n t ) ~ ] ~ - ~ , was observed to undergo only a single oxidation, Volts presumably producing [Co(mnt),],-. The polarographic behavior of [FeS4C4(CF3)4]2-2and [Fe(mnt)2]z-2 closely Figure 2. Polarograms in dichloromethane solution: (A) (nparallels that of their respective cobalt analogs. HowBu~N)z[COS~C~(CF~)~IZ, (B) (EtaN)[CoS4CdCFd4lz. The voltage scale is common to both polarograms; the left id/C scale refers to A. ever, on a rotating platinum electrode the reduction waves of these species, which should by analogy with the cobalt series produce [Fe-S4]+, are very drawn out Further, diffusion currents for the process [ C O - S ~ ] ~ - ~ and irreversible in either dichloromethane or acetonitrile. ~ [ C O - S ~are ] - ~roughly twice those for the single elecThe reason for this behavior is not understood. With a tron transfers. Because [CO-S~]-~species are unquesdme in acetonitrile this wave is well developed for tionably m o n o m e r i ~ , ~the ~ ~relative 7 values of the diffu[FeS4C4(CF3)4]2-2,but with [Fe(mnt),],-' an apparent sion currents are in agreement with the formation of reaction with mercury precludes measurement. dimeric species of charges -2, - 1, 0 in the redox reIt is to be noted that diffusion currents given in Table actionsm39 IV for redox reactions measured in solutions prepared from [M-S4]2-2g-1 salts or [M-S4l20have been calculated (39) Spectrophotomeric studies of the composition of solutions prepared from salts of the dimeric cobalt and iron dianions are curusing dimeric formula weights. These values in general
t
(38) The small variations in D j 2 observed are probably due to an incompletely compensated iR drop which results from working in a low dielectric solvent.
i:I
rently in progress. Of the species thus far studied in detail, it is concluded from Beer's law deviations that in dichloromethane [CoStC4(CFa)d]z-z and [Fe(mnt)~]~-Zare partially dissociated into monomeric monoanions. [F~S~CI(CFS)~]Z-~ is only very slightly dissociated. These observations refer to a 10-2-10-6 M concentration range of total metal
Balch, Dance, Holm
Dimeric Dithiolene Complexes
1144 Table V. Polarographic Data for Monomeric Bisdithiolene Complexes in Dichloromethane" ,b idC,
Metal
Ligand
Step
CU
SzC*(CN)z SZCz(CF3)z
- 2 z -1 -2 Jr - 1 -1 J - O d -2 -1 -I Z O d -2 Jr -1 -1 J - o d -2 -1 -1 J r o - 1 Jr 0' -1 J_ 0' 0 s +ld - 2 z -1 -1 I _ o -2 I _ -1 -2 -1 -1 ,-0 -2 I _ -1 -1 J_o
AU Ni
SzCz(CF3)z S2C2(CN)2 SzC?(CFa)z SzCCH3CH3 SzCz(CsHs)z SzCzHz
Pd
Pt
SzCz(CN)z SzCz(CF3)z SzCz(CFJ2
Eliz,v
!-ialmM
+0.41 0.00 +1.37 -0.97 +1.32 +0.37 +1.38
Z
-0.05
+1.01 +O. 52 +0.20 +1.45
-0.72 +0.28 $0.59 $0.25
+I .08 +0.02
$0.93
20
24 24 26
other iron complexes, a drawn-out reduction wave is observed on a platinum electrode, but this wave is considerably sharper in acetonitrile on a dme. The foregoing polarographic evidence leads to the following general formulation of the electron-transfer series of dimeric dithiolene complexes in dichloromethane solution. 2[M-S4]-2
25
21 24
21 23 22 25 21 27 28 20 19 24 18 23
a Using the exact method as described in ref 31. The potentials measured in this way differ by only 10.02 v from potentials measured using the modified reference cell described in the Experimental Section of this paper. b The following complexes were also investigated and found to show no clean oxidative waves: [Zn(mnt)*]-Z, [C~(tdt)~]-,c [Au(mnt)$ No attempt was made to study the reduction of these complexes in dichloromethane although all show well-defined waves in other solvents for the reduction to complexes with -2 charge. dNew waves which have not been observed in conventional polarographic solvents.
e [M-S~]Z-~e [M-SaIz- e [M-SaIzo
The entire situation is summarized in Table VI, which sets out the various series, partial and complete, which have been established polarographically together with the members of each which have been isolated and definitely characterized. As with monomeric complexes, the half-wave potentials of a given reaction are strongly ligand-dependent with the usual order of oxidative stability, tdt < SzCz(CF& < mnt, preserved for iron and cobalt. The large positive potentials required to form the neutral mnt dimers precludes their detection. The cobalt and iron dithietene series have been completely defined polarographically. The cobalt series is completely specified chemically as well because all four members have been isolated and characterized. Previous polarographic studies of several of the complexes studied here in dichloromethane did not lead to the recognition of dimeric species in acetonitrile solution, l 9 t 2 0 The polarographic behavior of the complexes formulated as [M-S,],-w (M = Fe, Co; n = 1, 2 ) has been reexamined in acetonitrile. Although the potentials of reduction waves observed correlated with those mea-
Table VI. Electron-Transfer Series Involving Dimeric Dithiolene Complexeso
Reference Species underlined have been isolated; they were characterized in the indicated references. Reference 20. This work. 19. reference 21. {Reference 12b and A. Davison, N. Edelstein, R. H. Holm, and A. H. Maki, J. Am. Chem. SOC.,85, 3049 (1963). Correctness of the monomeric formulation is uncertain. 0 Reference 30. h Detected by polarography in dichloromethane but not isolated.
In the absence of any information to the contrary, [Co(tdt)& and [Fe(tdt)& are taken to be unassociated in dichloromethane; both are monomers in acetonitrile (vide infra). In this solvent [Co(tdt)& does not exhibit any clean polarographic oxidation although reduction to [ C ~ ( t d t ) ~ ]has - ~ been observed.30 The iron analog undergoes two polarographic oxidations with diffusion currents consistent with the successive formation of [Fe(tdt)&- and [Fe(tdt)z]z. As has been found with the in all forms. The presence of monomers under conditions approaching those used in the polarographic studies does not invalidate the diffusion current values calculated on the basis of only dimers in solution, or the conclusions made above relative to the existence of dimeric electrontransfer series. A rapid monomer dimer equilibrium can accommodate consumption of the dimer in the electrode reaction, allowing all of the solute access to the electrode in dimeric form at those potentials under which the dimer, but not the monomer, is electroactive. That the monomers are not electroactive within potential ranges required to span the dimeric electron transfer series is indicated by the following observations: (i) three polarographic waves for most systems whereas monomeric species generally produce only two (cy, Table V), (ii) a ratio.oP -2 :1 :1 for diffusion currents of successive oxidative processes whereas an approximate 1 :1 ratio is found for monomeric complexes, (iii) balms of the diffusion currents for the two most positive oxidations which if calculated from monomeric formula weights are a factor of -2 too small for a transfer of one electron per metal.
*
Journal of the American Chemical Society
/
90:5
sured in dichlor~rnethane,~~ in only two cases were the anodic processes similar. Polarographic oxidation of [Fe(tdt)z]- in acetonitrile shows waves corresponding to the oxidation to [Fe(tdt)&- and [Fe(tdt)&. The previously reportedz0oxidative wave at Ell2 = 0.535 v for [ C O S ~ C ~ ( C F ~in) ~acetonitrile ]~-~ has a diffusion current which is half the value of the diffusion current of the cathodic wave at E l / , = -0.398 v. The anodic wave results in the production of [ C O S ~ C ~ ( C F ~ ) not ~]~-, [CoS4C4(CF3),]as previously reported; 2O Oxidation to [ C O S , C ~ ( C F ~is) ~not ] ~ detected in acetonitrile. The difference in polarographic behavior in dichloromethane from that in other solvents is probably due to two effects: (i) the ability of basic solvents to rupture the dimeric ions by coordinating with the (ii) the ability of basic solvents to reduce the oxidized member of couples with E,/, (in dichloromethane) greater than (40) For a large number of dithiolene complexes that exhibit similar behavior in the two solvents, the half-wave potentials are related by Ellz (in CHICN) E 0.88Ei/, (in CHZCIZ)- 0.15. (41) The polarography of five- or six-coordinate monomeric complexes is quite distinct from that of the dimers: A . L. Balch, Inorg.
Chern., 6 , 2158 (1967).
/ February 28, 1968
1145
+0.4 V . ~ O In connection with the latter suggestion, it should be noted that the two complexes whose polarographic behavior is similar in acetonitrile and dichloromethane are the cases where the least positive halfwave potentials are involved. In an effort to search for other examples of dimerization, the polarography of a number of other planar dithiolene complexes has been examined, The results are set out in Table v. Although no other dimeric species were found, several oxidative waves were observed which were undetected in other solvents. Dichloromethane appears to be an excellent, unreactive solvent for anodic polarography. Conductivity Results. The degree of association n of an electrolyte A,B, may be determined from measurement of the equivalent conductivity as a function of concentration. 4 z In this manner cobalt-mnt and -dithietene complexes were formulated as [Co(mnt)~]2-~ and [ C O S ~ C ~ ( C F ~ )and ~ ] ~[ C - ~~,( t d t ) ~ was ] - shown to be u n a s s o ~ i a t e d . ~ 7 , Spectral ~~ results39 are consistent with this observation for the first complex, but for the second they reveal -50% dimer dissociation in the lower concentration range employed in the conductivity study. Evidently the published data2' are insufficiently accurate to detect the substantial degree of dissociation, Results of similar studies of [Fe-S4],-, ions in acetonitrile are given in Table VI1 and presented graphically
90 00 70
-
-
50 40 60
002
004 006
0 0 0 010
0.12
fi Figure 3. Conductivity results for electrolytes in acetonitrile solution: 0 , (n-BuaN)[Fe(mnt)z]; 8 , (n-BuJT)[Fe(tdt)r]; - . -, (n-Bu4N)[Ni(mnth]; - - - -, (n-BuaN)[Co(tdt)a]; - - -, (nBu4N)dNi(mnt)~l.
[Re(mnt)2]z-z,4 4 in which the strong Re-Re bond prevalent in binuclear Re(II1) species may be preserved, Compound A, ohm-' cm2 epuiv-1 b A The low solubilities of [MS4C4Ph4Iz6 (M = Fe, Co) and [Sn(tdt)2]45could be accounted for by six-coordinate polymeric structures formed by repetition of the dimeric unit. Salts containing, nominally, [FeS4C4Ph4]and [CoS4C4Ph$ have recently been prepared. z6,46 The reportedz6diamagnetism of the tetraethylammonium 'a C ref 27 for additional results on the same and related comsalt of each requires a dimeric or polymeric structure for plexes. b Cf.ref 43. the iron complex and is suggestive of the same for the cobalt complex. The dimer structure is not confined in Figure 3. A plot of conductivity us. d 2 (c = 10-2to dithiolene complexes. Extremely similar M-S5 col e 3 M ) for (Bu4N)2[FeS4C4(CF3)4]2 (not shown) in ordination polyhedra are found in crystalline bis-Nacetonitrile is nonlinear, consistent with some dimer N-diethyL47 and bis-N,N-di-n-propyldithiocarbamatodissociation. With the adoption of (B~~N)z[Ni(mnt)~]~ o p p e r ( I I ) ,which ~~ exist as centrosymmetric dimers. and ( B ~ ~ N ) [ N i ( m n as t ) ~standard ] 2: 1 and 1: 1 electrolytes This same structure may extend to M(S2CNEt& comand the established monomeric nature of (Bu4N)[Coplexes isomorphous with Cu(S2CNEt&. 4 9 (tdt)J, 2 4 ~ z 7 the most reasonable interpretation of the Acknowledgment. This research was sponsored by results in Figure 3 is that (BudN)[Fe(tdt)n] and (Bu4N),the National Science Foundation under Grant GP[Fe(mnt)z]zyield monomeric anions in acetonitrile over 4899. We thank Drs. I. Bernal, R. Eisenberg, and H. the concentration range studied. The spectrum of the B. Gray for communication prior to publication of Xsecond compound in acetonitrile, when compared with ray structural results, Dr. A. Davison for permission those of a dichloromethane solution and the solid, is t o reproduce Figure 1, and Dr. J. A. McCleverty for consistent with complete dimer dissociation. The access to the results given in ref 46 prior to its publicaspectrum of crystalline (Bu4N)[Fe(tdt)z] also changes on tion. dissolution in acetonitrile. However, the detailed state (44) F. A. Cotton, C. Oldham, and R. A . Walton, Inorg. Chem., 6 , of association of [Fe(tdt)2]- in the solid and in solution 214 (1967). where its spectrum is markedly solvent dependent30 is (45) R. C. Poller and J. A. Spillman, J . Chem. SOC., Secr. A , 958 (1 966). not yet clear and must await the results of further in(46) J. A . McCleverty, N. M. Atherton, J. Locke, E. J. Wharton, and vestigations in progress. C. J. Winscom, J . A m . Chem. SOC.,89, 6082 (1967). Other Sulfur-Bonded Dimers and Polymers. An (47) M. Bonamico, G. Dessy, A. Mugnoli, and A. Vaciago, Acta Cryst., 19, 886 (1965). exception to the dimer structure of Figure 1 may be Table VII. Conductivity Measurements in Acetonitrile Solution"
(42) R. D. Feltham and R. G . Hayter, J . Chem. SOC.,4587 (1964). (43) We have studied several compounds that were also studied in ref 27. Our values of A agree with those obtained by Davison, et al.:
however, our values of ha are uniformly lower. This discrepancy may result from errors in cell calibration which would affect A O but not A . Since the degree of association is determined from comparison of A values, the discrepancy does not effect any of the conclusions,
(48) A. Pignedoli and G . Peyronel, Gazz. Chim. Ita/., 92, 745 (1962). (49) J. P. Fackler, Jr., and D. G . Holah, Inorg. Nucl. Chem. Letters, 251 (1966). Note that the isomorphism of the Cu(II), Mn(II), and
Fe(I1) complexes with Cu(SzCNEt2)z does not certify that they all are isostructural; the Zn(I1) complex is also claimed to be isomorphous but has a centrosymmetric dimeric structure which is markedly different from that of Cu(SZCNEt2)2; cf. M. Bonamico, G . Mazzone, A . Vaciago, and L. Zambonelli, Acta Cryst., 19, 898 (1965).
Balch, Dance, Holm
/ Dimeric Dithiolene Complexes
